When you go to register/update a credit/debit card, do you keep getting error codes 601 and 401?
I was having the same issue and I finally got answers for those of you who are having the same, frustrating problem!

So that you know, the Square Enix Customer Service Number is +1.310.846.0400.

"601' and "401" Error Codes are described as a problem has occurred having to do with the rejection/decline of your card.
Before you call, double check the following:
1. Make sure your information matches what is on the card/ all information entered is correct.
2. Double check with your bank that you have sufficient funds for the basic service/character fees.

Now, if both list items above are fufilled, and you still recieve either codes (This could be the MAIN reason) = You may have been blocked by the site!
Think of it this way - If you try to enter your password for your email account too many times and it is incorrect, you get blocked and can no longer try!
This can be easily fixed by calling the above number ^ Tell them you recieve the codes and that YES you have double checked with your bank and you HAVE sufficient funds. Have them look further into the matter (if the person you get is new and doesn't know, have them switch you to someone higher up to confirm), if the case was that you tried inputting too many times, they can bypass the site block and allow you to try it again!

If it IS a bank problem, simply call /visit your bank and have them process the square enix request. (Sometimes that is all it is).

I hope this helped, I saw a lot of questions about this is different forums. Please comment any questions, or add to what I said! :) Thank you!